In toad urinary bladder, osmotic shock (application of distilled water in the serosal compartment versus amphibian ringer at the mucosal side) induces massive, fast and partially reversible proliferation of tight junction strands. Two minutes after initiation of the osmotic shock, the proliferation process is 75% complete at 37 degrees C (40% at room temperature). Maximum-proliferation is reached after 10 min at 37 degrees C (60 min at room temperature). Cytoskeleton perturbers (colchicine, cytochalasin), as well as cycloheximide (an inhibitor of protein synthesis) have no effect on he proliferation process. Limited reversal of junctional strand proliferation is a slow process, accelerated by the presence of cycloheximide.